1. Field of the Invention
This invention relates to an iron-based sintered component formed of an iron-based metal powder as a raw material and suitable to machinery parts, or an iron-based powder metal body as an intermediate material suitable to manufacture of the sintered iron-based component.
2. Description of the Related Art
Powder metallurgical technology can produce a component having a complicated shape as a xe2x80x9cnear net shapexe2x80x9d with high dimensional accuracy and can markedly reduce the cost of cutting and/or finishing. In such a near net shape, almost no mechanical processing is required to obtain or form a target shape. Powder metallurgical products are, therefore, used in a variety of applications in automobiles and other various fields. For reduction in size and weight of the components, demands have recently been made on such powder metallurgical products to have higher strength. Specifically, strong demands have been made on iron-based powder products (sintered iron-based components) to have higher strength.
A basic process for producing a sintered iron-based component (sometimes hereinafter referred to as xe2x80x9csintered iron-based compactxe2x80x9d or simply as xe2x80x9csintered compactxe2x80x9d) includes the following sequential three steps (1) to (3):
(1) a step of mixing sub-material powders such as a graphite powder and/or copper powder and a lubricant such as zinc stearate or lithium stearate to an iron-based metal powder to yield an iron-based powder mixture;
(2) a step of charging the iron-based powder mixture into a die and pressing the mixed powder to yield a green compact; and
(3) a step of sintering the green compact to yield a sintered compact.
The resulting sintered compact is subjected to a sizing or cutting process according to necessity to thereby yield a product such as a machine component. When a higher strength is required for the sintered compact, it is subjected to heat treatment for carburization or bright quenching and tempering.
The resulting green compact obtained through the steps (1) to (2) has a density of at greatest from about 6.6 to about 7.1 Mg/m3 and, accordingly, a sintered compact obtained from the green compact has similar density.
In order to further increase the strength of such iron-based powder products (sintered iron-based components), it is effective to increase the density of the green compact to thereby increase the density of the resulting sintered compact obtained by subsequent sintering. The component has fewer voids and better mechanical properties such as tensile strength, impact resistance and fatigue strength when the sintered compact has a higher density.
A hot pressing technique, in which a metal powder is pressed while heating, is disclosed in, for example, Japanese Published Unexamined Patent Application No. 2-156002, Japanese Published Unexamined Patent Application No. 7-103404 and U.S. Pat. No. 5,368,630 as a pressing process for increasing the density of a green compact. For example, 0.5% by mass of a graphite powder and 0.6% by mass of a lubricant are added to a partially alloyed iron powder in which 4 mass % Ni, 0.5 mass % Mo and 1.5 mass % Cu are contained, to yield an iron-based powder mixture. The iron-based powder mixture is subjected to the hot pressing technique at a temperature of 150xc2x0 C. under a pressure of 686 MPa to thereby yield a green compact having a density of about 7.30 Mg/m3. However, application of the hot pressing technique requires heating facilities for heating the powder to a predetermined temperature which increases production cost and decreases dimensional accuracy of the component due to thermal deformation of the die.
Further, Japanese Published Unexamined Patent Applications No. 1-123005, for example, discloses sintering cold forging process as a combination of the powder metallurgical technology and cold forging that can produce a product having a substantially true density.
The sintering cold forging process is a molding/working method for obtaining a final product of high density composition by compacting a metal powder such as an iron-based powder mixture into a preform, preliminarily sintering the preform, cold forging and then re-sintering the same instead of the steps (2) and (3) described above. In this invention, the preliminarily sintered body is particularly referred to as a (iron-based) sintered powder metal body. Further, when it is referred to simply as a sintered body or sintered component, it means a sintered body obtained by re-sintering and/or heat treatment. The technique described in Japanese Published Unexamined Patent Application No. 1-123005 is a method of coating a liquid lubricant on the surface of a preform for cold forging and sintering, provisionally compacting the preform in a die, then applying a negative pressure to the preform to thereby suck and remove the liquid lubricant and then re-compact and re-sinter. According to this method, since the liquid lubricant coated and impregnated to the inside before the provisional compaction is sucked before the re-compaction, minute voids in the inside are collapsed and eliminated during re-compaction to obtain a final product with high density However, the density of the final sintered product obtained by this method is about 7.5 Mg/M3 at the greatest and the strength has a limit.
For further improving the strength of the product (sintered body), it is effective to increase the concentration of carbon in the product. It is general in the powder metallurgy to mix a graphite powder as a carbon source with other metal powder materials, and it may be considered a method of obtaining a high strength sintered body by compacting and then preliminarily sintering a metal powder mixed with a graphite powder to form a sintered preform, further re-compacting and re-sintering (application of sintering/cold forging method). However, when preliminary sintering is applied in the existent method, about all of the mixed carbon diffuses into the matrix of the preform upon the preliminary sintering to increase the hardness of the sintered powder metal body. Therefore, when the sintered powder metal body is re-compacted, the re-compacting load increases remarkably and the deformability of the sintered powder metal body is lowered, so that it can not be fabricated into a desired shape. Accordingly, high strength and high density product can not be obtained.
For the problem described above, U.S. Pat. No. 4,393,563, for example, discloses a method of manufacturing a bearing component without pressing at high temperature. The method comprises the steps of mixing an iron powder, an iron alloying powder, a graphite powder and a lubricant, compacting the powder mixture into a preform, preliminarily sintering and then subjecting the same to cold forging with at least 50% plastic working, then re-sintering and annealing and roll forming the compact into a final product (sintered component). For the technique described in U.S. Pat. No. 4,393,563, it is described that when preliminary sintering is applied under the condition of suppressing diffusion of graphite, the preliminarily sintered component (preliminarily sintered body) has high deformability and can lower the compacting load in the subsequent cold forging. U.S. Pat. No. 4,393,563 recommends preliminary sintering conditions of 1100xc2x0 C.xc3x9715-20 min. However, it has been found by the experiment of the present inventors that, under the conditions described above, graphite is completely diffused into the preform to remarkably increase the hardness of the material for sintered preform to make the subsequent cold forging difficult.
For the problem described above, Japanese Published Unexamined Patent Application No. 11-117002 proposes, for example, a sintered powder metal body by compacting a metal powder formed by mixing 0.3% having a structure where graphite remains at the grain boundary of the metal powder by weight or more of graphite with a metal powder mainly comprising iron to obtain a preform having a density of 7.3 g/cm3 or more, and preliminarily sintering the preform within a temperature range, preferably, from 700 to 1000xc2x0 C. According to this technique, since only the amount of carbon required for increasing the strength is solid solubilized by the preliminary sintering within the temperature range as described above to leave free graphite and prevent excess hardening of the iron powder, compacting material (sintered metal body) having low compacting pressure and high deformability can be obtained upon re-compaction step. However, although the metal powder compacting material (sintered powder metal body) obtained by this method has a high deformability in the re-compaction step, remaining free graphite is eliminated in the subsequent re-sintering to yield elongate voids (pore) to possibly lower the strength of the sintered product.
This invention intends to overcome the foregoing problems in the prior art and provide, at first, an iron-based sintered powder metal body capable of manufacturing a compact with outstandingly lower re-compacting load having outstandingly higher deformability compared with the prior art and having a high density upon manufacturing a powder metallurgical product starting from the iron-based powder mixture, as well as a manufacturing method thereof.
This invention also intends to provide a method of manufacturing an iron-based sintered body with fewer voids of a sharp shape and having high strength and high density.
In order to attain the subject described above the present inventors have made an earnest study on the compaction and preliminary sintering conditions. As a result, it has been found, for suppressing the occurrence of elongate voids, that it is effective to compact the iron-based powder mixture to a high density and, further, preliminarily sinter the same at a temperature enough to diffuse the added graphite into the matrix thereby reducing the amount of free graphite to substantially zero. Further, for remarkably decreasing the hardness of the sintered metal body even when the preliminary sintering is applied at such a temperature, it has been found to be effective that the nitrogen (N) content in the iron-based sintered powder metal body is reduced and, further, annealing is conducted succeeding to the preliminary sintering or the preliminary sintering is condacted in an atmosphere of suppressing nitridation. This can attain a low load upon re-compaction and can provide high density compact and, as a result, a sintered body of high density and high strength can be manufactured.
This invention has been accomplished by a further study based on the findings as described above.
That is, this invention relates, at first, to an iron-based sintered powder metal body the density of which is about 7.3 Mg/m3 or more and which comprises, on the mass % basis, at least about 0.10% and at most about 0.50 of carbon and at most about 0.3% of oxygen and at most about 0.010% (preferably about 0.0050%) of nitrogen, and which comprises at most about 0.02% of free carbon, obtained by compaction and preliminarily sintering an iron-based powder mixture prepared by mixing an iron-based metal powder, a graphite powder and, optionally, a lubricant.
Another invention relates to a method of producing an iron-based sintered powder metal body comprising the steps of mixing at least,
an iron-based metal powder comprising, on the mass % basis,
at most about 0.05% of carbon,
at most about 0.3% of oxygen,
at most about 0.010% (preferably about 0.0050%) of nitrogen, with at least about 0.03% and at most about 0.5% of graphite powder based on the total weight of the iron-based metal powder and the graphite powder and, optionally, at least about 0.1 weight parts and at most about 0.6 weight parts of lubricant based on 100 weight parts of total weight of the iron-based metal powder and the graphite powder, resulting in an iron-based powder mixture, compacting the powder mixture into a preform, the density of which is about 7.3 Mg/m3 or more, and preliminarily sintering the preform in a non-oxidizing atmosphere in which partial pressure of nitrogen is about 30 kPa or less and at a temperature of about 1000xc2x0 C. or higher and about 1300xc2x0 C. or lower.
As embodiment of another invention may adopt a method of manufacturing an sintered iron-based powder metal body comprising preliminarily sintering the preform at a temperature of about 1000xc2x0 C. or higher and about 1300xc2x0 C. or lower and then annealing the same. The atmosphere in the preliminary sintering has no particular restriction but it is preferably conducted in a non-oxidizing atmosphere at a nitrogen partial pressure of about 95 kPa or lower. Further, annealing is conducted preferably within a temperature from about 400 to about 800xc2x0 C.
A further invention provides a method of manufacturing a high strength and high density iron-based sintered body comprising re-compacting the iron-based sintered powder metal body obtained by each of the methods of another invention and then re-sintering and/or heat treating the compact.
In each of the inventions described above, the composition for the iron-based sintered powder metal body or the composition for the iron-based powder mixture further contains, preferably, one or more of elements selected from the group consisting of, at most about 1.2% of manganese, at most about 2.3% of molybdenum, at most about 3.0% of chromium, at most about 5.0% of nickel, at most about 2.0% of copper, and at most about 1.4% of vanadium each on the mass % basis. The form of containing the alloying elements (Mn, Mo, Cr, Ni, Cu, V) in the iron-based metal powder has no particular restriction. It may be a mere mixture of an iron-based metal powder and an alloying powder but it is preferably a partially alloyed steel powder in which the alloying powder of the alloying elements described above is partially diffused and bonded to a surface of the iron-based metal powder. Further, pre-alloyed steel powder containing the alloying elements described above in the iron-based metal powder itself is also preferred. The forms of containment described above may be used in combination.
Further, in each of the inventions described above, for the composition of the iron-based sintered powder metal body or the composition for the iron-based powder mixture described above, other ingredients than those described above are not particularly restricted so long as most of the remainder (about 85% or more) is iron, and a composition comprising the remainder of Fe and inevitable impurities is preferred.